Powder spray systems are well known in the art and representative systems are disclosed, for example in U.S. Pat. Nos. 4,910,047 and 4,723,505, both assigned to Nordson Corp. of Westlake, Ohio, the assignee of this invention. As disclosed in these patents, the practice of powder coating involves spraying a powdered coating material onto a substrate and thereafter heating the substrate and the powder so that the powder melts. When subsequently cooled, the melted powder forms a solid, continuous coating on the substrate. In many instances, an electrostatic charge is applied to the sprayed powder and the substrate is electrically grounded to increase the quantity of powder which attaches to the substrate and to assist in retaining the powder thereon.
Powder deposition is usually performed in a spray booth, i.e., an enclosure wherein any oversprayed powder which is not deposited on the substrate can be collected. Conventionally, the containment of oversprayed powder in the spray booth is aided by an exhaust system which creates a negative pressure within the spray booth and draws the powder entrained in a stream of air out of the spray booth into a powder recovery unit, also called a powder collector. In the powder collector, the particles of powder are separated from the air by a filter media, collected in a hopper and then usually returned to the powder supply for sieving and recirculating to the spray gun.
Early U.S. Pat. No. 3,870,375 shows the use of filter bags for the filtering media. Typically, multiple filter bags were contained in a bag house in these systems. The openings in the top of the bags were connected to a clean air plenum which is kept under negative pressure by an exhaust fan. In this way, as air entrained powder is drawn towards and contacts the bag, the powder would remain on the bags exterior, while the filtered air would flow into the interior of the bag, into the clean air plenum, and then through the exhaust fan, and final filter.
In the 3,870,375 patent, an oversized bag house is employed which permits one entire section of the bag house to be taken off line (i.e. disconnected or sealed off from the exhaust fan) at any given time without disrupting the ability of the bag house to continue to draw enough oversprayed powder into the collector so that the powder does not start to escape through the booth openings into the work environment. With a section of the bag house sealed off from the exhaust fan, the bags in that section would not be under the influence of the negative pressure, or draw from the exhaust fan, and the bags could be mechanically vibrated to dislodge powder collected on the exterior of the bags so that the dislodged powder could be collected in the collection hopper underlying the bags. In this type of system, it was necessary to completely seal the bags off from the exhaust fan during cleaning, because if the bags felt even a small draw from the fan the powder would be very difficult to dislodge by vibration.
U.S. Pat. No. 4,218,227 shows and describes the replacement of bags by cylindrical, pleated paper cartridge filters. With the advent of cartridge filters, powder collected on the exterior of the filters was knocked off not by mechanical vibration per se, but instead by a brief pulse of air directed down into the interior of the cartridge in the reverse direction to the normal filtered air flow through the cartridge caused by the exhaust fan. In these systems, the reverse cleaning pulse would momentarily overcome the exhaust fan air flow through the cartridge to knock the powder off the exterior of the cartridge and down into the collection hopper. Typically, cartridges are cleaned in sequence, one or two at a time, so that the overall ability of the collector to collect oversprayed powder from the booth and keep it from escaping from the booth into the work environment is not impaired.
A problem with this type of prior air, pulse cleaning system, however, is that all the cartridge filters are connected to a common clean air chamber which is constantly under the force of the exhaust fan to draw air entrained with oversprayed powder through the filters, even while the filters are being pulse cleaned. This causes some particles of powder which were just cleaned off the outer surface of a cartridge filter by a high pressure pulse to be drawn right back onto the cartridge following the end of the pulse. Also, the particles of powder which are cleaned from one cartridge are sometimes re-deposited onto a neighboring cartridge. The consequence of this problem is that the cartridges do not get a thorough cleaning, the pressure of the pulse air has to be maintained at a high level which increases the pulse noise of the system and the cartridges have to be pulse cleaned more often.
More recently, U.S. Pat. No. 5,002,594 issued which attempts to improve upon cartridge filter systems by enclosing the top of the cartridge and the pulsing mechanism by a noise shield during pulsing to reduce the noise level of the pulse. While this design may reduce the noise level of the pulse, it blocks air surrounding the top of the cartridge from being entrained with the pulse which according to the teachings of the present invention is a serious drawback to this design. The reason for this is that according to the present invention, it is necessary to permit air surrounding the top of the cartridge to be entrained with the pulse to generate a high enough volume pulse to effectively dislodge powder from the cartridge exterior. Moreover, by completely sealing the top of the cartridge, or even the area of the air plenum above the.TM.top of the cartridge, not only is air entrainment prevented or retarded, but in addition, a vacuum, or partial vacuum condition can be set up in the cartridge or air plenum during pulsing which works against the pulsing force and makes the pulse less effective.
Another problem with the cartridge pulsing system of the 5,002,594 patent, as with the other prior art cartridge pulsing systems as noted above is that powder particles pulsed off one cartridge can be immediately drawn onto an adjacent cartridge requiring the cartridges to be cleaned more frequently.
Aside from the factors relating to the pulse cleaning of the cartridges, another factor in the design of powder collectors is the manner in which the cartridge filters are mounted within a powder recovery chamber. Cartridge filters must be periodically replaced because of the gradual deterioration of the filters and filter leaks caused by the substantial forces generated by the pressures and flow rates of the air in both directions through the cartridge. The accessibility and ease of replacement of the filters is therefore a very important consideration. Further, when a filter is installed, it must be tightly mounted so that no air leaks exist between the collection chamber containing the unfiltered, particle entrained air and the clean air chamber. Conversely, the filter cannot be mounted so tightly that the filter cartridge or gasket at either end thereof is overcompressed and damaged to create an air leak. When particles of powder escape through an air leak into the clean air plenum, any downstream filter, specifically the final filter which is designed to filter extremely small sized particles, quickly clogs causing a shutdown of the entire powder coating operation.
The prior art cartridge filters were sometimes mounted more or less horizontally within a powder recovery chamber, as disclosed, for example in U.S. Pat. No. 4,395,269, U.S. Pat. Nos. 4,723,505 and 4,871,380.
Other cartridges have been vertically mounted, as disclosed, for example in U.S. Pat. Nos. 4,910,047 and 4,955,996. Typically, vertically mounted filter cartridges, are mounted from their top flange, as disclosed in the 4,910,047 and 4,955,996 patents or held in compression by means of a bolt extending through a hole in the bottom end cap of the cartridges as shown in FIG. 4 of U.S. Pat. No. 4,218,227.
A problem with the prior art, vertically mounted cartridge filters is the loss in cartridge surface area available for air flow and reverse jet pulsing at the very top of the cartridge due to the placement of structural components at the top opening of the cartridge which are required for supporting the filter vertically. These structural components have blocked or shadowed the pulse at the top of the cartridge preventing it from being effectively cleaned. Consequently, additional filter cartridges were required to compensate for this loss of filter surface area. Also, the installation of the prior art, vertical filters has typically been a tedious, frustrating process especially for a single installer. Accordingly, the installation typically required two installers, one on the side of the clean air chamber and the other on the spray booth side of the powder collector.
Another problem with the installation of the prior art, vertically mounted cartridge filters has been the tendency of the installer to over tighten the bolt extending through the cartridge with the result that the filter cartridge becomes deformed or otherwise damaged causing a leak.